This invention relates to control circuits and methods for paralleling AC electrical power systems and, more particularly, to such circuits and methods which are applicable to parallel AC power systems having channels with different power ratings.
AC electric power systems are generally connected in parallel to increase total system rating, or in certain cases such as airborne power systems, to increase reliability. Typical aircraft AC electrical power systems include two or more identical power channels. These channels may include a constant speed generator with a constant speed drive (CSD) for frequency control, or a variable speed generator with a constant frequency electronic inverter (VSCF). To improve reliability and maximize efficiency, it is generally desired that the total system load be divided equally among the paralleled generators. Load division among AC generators is accomplished by controlling, in a closed loop manner, individual generator voltages and phase angles.
Other types of electrical power systems which may be operated with parallel channels include photovoltaic, thermionic, fuel cell, or battery systems with inverter outputs such as used for space applications. All of these systems require active control of both real and reactive power flow between the paralleled channels. Controls for paralleling AC systems have been developed for paralleled AC electrical systems. These controls use currrent transformer loops to sense difference currents between channels. Frequency and voltage controls then drive the difference currents to zero. This type of control system will also work between channels of different power rating if appropriate current transformer ratios are chosen.
Advance electrical AC power systems will require that the generating channels have a programmable power rating. Programmability will be needed to accommodate degraded power sources caused by failed rotating rectifiers in brushless AC generators, reduced cooling capacity, damaged fuel cells, etc.